This invention relates to recycling of metal matrix composites and more particularly, to a process for removing contaminants from scrap metal matrix composites, such as gates and risers from the casting of aluminum matrix composites.
By scrap metal matrix composites is meant material in which the degree of wetting of the particulate reinforcing media by the aluminum matrix remains unimpaired. Such otherwise sound material may be contaminated by, e.g. oxide film inclusions which decrease the fluidity of remelted scrap material and the mechanical properties of the product cast therefrom.
Aluminum matrix composites are widely used where high strengths and other enhanced properties are required in casting. As materials for reinforcing the aluminum matrix, a variety of non-metallic particulates are typically used, e.g. silicon carbide, alumina, etc. These aluminum matrix composites may contain hydrogen in gaseous form as well as oxide inclusions, aluminum carbide, etc. and the presence of these contaminants represents an ongoing problem for foundries involved in casting of such composites. Such contaminants have detrimental effects on the fluidity of molten composites and mechanical properties of the cast products.
A particularly troublesome species of these contaminants are oxide films or inclusions. These oxide films have approximately the same density as an aluminum matrix composite melt because of the agglomeration of particulates on the oxide films. The presence of oxide films in the melt has a very detrimental effect on the ultimate tensile strength and elongation of the cast product and, furthermore, these oxide films drastically affect the fluidity of the melt, which is one of the most important physical properties of a casting alloy.
The injection of an inert gas or inert-reactive gas mixtures into molten metal is a commonly used technique for the removal of the above contaminants. Such systems are described in Bruno et al, U.S. Pat. Nos. 3,839,019, Szekely, 3,743,263, Withers et al, 4,634,105 etc. These prior systems are based upon injecting gas in the form of small discrete bubbles and these small bubbles are intended to float or dewet a substantial proportion of any solid particulates contained in the molten metal and remove other contaminants, such as dissolved hydrogen, in the melt. In the case of composites, the gas bubbles end up removing the composite reinforcing particles thereby defeating the purpose for aluminum matrix composites.
Various attempts have been made to solve the problem of removing contaminants from aluminum matrix composites while leaving the reinforcing particles in place. Such attempts have included trying to carefully control the mixing of the molten composite such that oxide inclusions are not generated and the gas bubbles have been introduced as large discrete bubbles which were reasonably successful in avoiding any substantial flotation or dewetting of the reinforcing particles such that they rose to the surface of the melt. However, the large gas bubbles tended to burst at the surface of the melt and this resulted in oxide films or inclusions being drawn back into the melt. This had a disastrous effect on the fluidity of the melt, changing it to a thick paste consistency.
It is the object of the present invention to provide a process for selectively removing unwanted contaminants from aluminum matrix composites without also removing the particulate materials added as reinforcing agents and without introducing oxide films or inclusions into the composite, e.g. for recycling otherwise sound scrap material, such as foundry gates and risers, to restore fluidity and mechanical properties.